In its first month of operations, Larkspur, Inc. made three purchases of merchandise in the following sequence: $(1)(2)480$ units at $$8,$ and $(3)105$ units at $$9.$

$($a$1)$

Calculate the weighted$-$average unit cost. $($Round answer to $3$ decimal places, e$.$g$.15\mathrm{.}225.)$

Weighted$-$average unit cost $